Television signal control circuit



April 11,V 1944.

Z/ PRE MPL/HER HMPL/F/ER 7/ PED/55741. ccm/m01 Z7/0DE 5 Ol/TPUT MPL/Hm R. E. RUTHERFORD ETAL TELEVISION SIGNAL CONTROL CIRCUIT Filed Dec. 18, 1941 2 Sheets-Sheet l 'ffz -/17' l I I7l T f I116 @ff April 1l, 1944. R. E. RUTHERFORD ET AL 2,346,509

TELEVISION SIGNAL CONTROL CIRCUIT Filed Dec. 18, 1941 2 Sheets-Sheei'i 2 Z/IV 1:24: 4. www@ 5 INVENToRs Patented Apr. 1l, 1944 TELEVISION SIGNAL CONTML CIRCUIT Robert E. Rntherl'ordRutherf0rll, and 'Richard L. Campbell,

Maywood, N. J.,

assignors to Allen B. Du Mont Laboratories, Inc., Passaic, N. J., a corporation of Delaware ApmiiiOn'neeemm 1s, 1941,' serial No. 423,460

(ci. 11s- 1.2i

4-Clain.

This invention .relates to a television transmission system in'which clipping and limiting circuits are provided. Linear amplication of the video signals takes place and abrupt limitation of signal level is provided to prevent video components from getting above the blanking level into the synchronizing region. Means are also provided for controlling the pedestal level and for inserting direct current components in the television signal.

In carrying out this invention a blanking signal is applied to the amplified signal from the pick-up camera, the pedestals thus formed are clipped to form flat topped pedestals, the signal is amplied, and the pedestals are again clipped and amplified. A diode tube is used for clipping, and linear response below the clipping level can be maintained at all times with this invention. The clipping level can be readily and accurately'adjusted by varying the direct current bias, the pedestal height is eectively controlled after clipping, and the invention will operate over a very wide range of signal levels.

In the conventional system of television transmission it has been customary to arrange the receivers with automatic background control for the average brilliance of the pictures, facilities for too much light and adjust the signals to give the bestv over-all performance through the amplitude limitations of the transmitting and receiving system. The camera sensitivity, the lens aperture. and the ampliiler gain alsoneed to be adjusted to further balance the average drawings. in which:

being included for maintaining the cut-oil voltage of the cathode-ray tube at a point' corresponding to the black level ofA the composite synchronizing wave form. In .order for the scenes to'be received properly, it has been necessary for the transmitter to adjust the average illumination by means of suitable composition of the transmitted wave form. The average axis of the useable picture wave form determines the average brilliance on the receiving tube with re spect to the black level. Consequently, it has heretofore been necessary to adjust the average level of the wave form at the transmitter with respect to the black level. This can be done by purposely shifting the black level to a new point so as to give a variation between the range of the black level and the average level of the useable picture signal. Heretofore it has been necessary to chang this range, particularly for l Fig. 1 is a block diagram showing an arrangement of apparatus used in the invention:

Figs. 1a.,.1b, 1c, 1d, 1e, 1f, and 1g are diagrams of signals at diierent stages;

Fig. 2 is a diagram having a signal associated therewith showing a diode for use in the inven-- tion;

Fig. 3 is a diagram showing a cathode-loaded stage driving a, clipping diode;

Fig. 4 is a diagram showing a plate-loaded stage driving a clipping diode; and

Fig. 5 shows two diodes for cascaded clipping of signals in a synchronizing generator.

In the drawings, reference character I in Fig. 1 indicates an iconoscope or television pick-up device, and the words in the blocks 2, 3, l, 5, 6, l and 8 of the diagram of Fig. 1 show the arrangement of different. devices in accordance with this invention.

Reference character II (Fig. 1a) indicates a video signal of the well known sort, and I2 in Fig. lbvindicates a blanking signal for the video signal Ii.

Reference character I3 in Fig. 1c indicates the video signal after the blanking signal I2 has been applied, the dotted line Il indicating where the pedestals I5 are to be 'clipped to form the sig- Inal I 6 as shown in Fig. 1d with the dat topped pedestals I'I. Fig. 1e shows the signal I6 of Fig. 1d amplified to I6' with the pedestals amplied as indicated at Il', and the dotted line I8 shows where this pedestal is to be clipped. The clipping results in the formation of the signal as indicated in Fig. 1f by the reference characters I9 and 20, the characters 20 representing the pedestals further clipped. The signal is nally ampliiied as indicated in Fig. 1g into the sig-A nfal i9'l with the pedestals 20'.

In Fig. 2 is shown, in more detail, the unit designated by the block diagram 5 in Fig. 1 and called a pedestal clipper diode. The mixed signal is indicated atthe top of this Fig. 2 by reference character 25, and consists of the desired ypicture component 26 and spurious signal 21 which is superposed upon the inserted blanking 28.l The dotted line 29 represents the level at which clipping will occur in the diode $30, this level being determined by the setting of the bias voltage by the movable arm 3| on the potentiometer 32. This signalwave form is fed into the plate 33 of the diode 30, which has a resistance 34 connected to ground. A load resistor 35 is in the circuit of the cathode 31 of the' diode 30, and the condenser 38 couples the clipped output signal to the output lead wire 39 on which appears the clipped wave form 40 consisting of the desired picture components 26 and the square topped clipped pedestals 4|. This diode 30 removes the extraneous wave form 21 and prepares the signal for passage to the remaining television equipment.

This diode clipper, which is used in the posi-- tion of block diagram 5 (Fig. l) not only removes the extraneous signals during the blanking interval, but, further, prevents any possibility of picture components extending up into the amplitude region which is to be devoted to synchronizing. It is very important that no signal wave forms reach into this region except the desired vsynchronizing impulses which are introduced later on to be mixed with the output Wave forms of Fig. 1f or Fig. lg to compose the well known composite television signal.

An additional clipper in the television amplier chain may also be used whenever desired to limit the excursions in the white direction so as to prevent overload of the television transmitter.

By use of these clippers, it is possible to transmit motion picture lm in' which the contrast range, and consequently the level of the picture signal amplitude, changes from low values to very high values, since the clippers will prevent the pictures from exceeding the useful range and encroaching upon the synchronizing level A0r encroaching upon the whiter than white level. This restriction of'amplitude is automatic and works far more rapidly than one could operate a manual gain control to restrict the output signal from the amplifier.

A modification of the invention is shown in Fig. 3, which includes a tube that is connected as a cathode follower stage to serve as an impedance transformer.4 The signal, such asI 25 above, is fed to the terminal 5| and passes through the condenser 52 to the grid 53 of the tube 50. The resistor 54 is inserted `between this grid and groundto hold the grid at the proper average voltage. In some cases this resistor 54 may be returned to a positive potential. The plate 55 and screen grid 56 of the tube 50 are connected to a positive supply voltage. The load circuit of the tube 50 consists of the resistor 58 inthe circuit of the cathode 59. Essentially the same signal appears on the lead from this cathode as on the lead 5|, except that this signal is driven by a much lower impedance which is capable of feeding the following diode 63 without distortion. The lead 60 from tube 50 is conbe clipped to yield a square topped signal similar to that shown in the wave form 40 of Fig. 2.

A further modification of the invention is shown in Fig. 4 in which the clipping diode is fed from a plate-loaded vacuum tube 15. The signal to be applied to the terminal 16 would in general bc the reverse polarity from the wave form shown by Fig. 2. It is fed through the condenser 11 to the grid 18 of the tube 15. The grid leak-resistor nected to the plate 64 of the diode 63, and the cathode 85 of this diode is connected through the load resistor 66 to a source of variable bias other end being connected to ground. The output signal is coupled by condenser 69 to the remainder of the television equipment. By adjust- -rnent of the slider 58 the signal fed in at 5| can l sion pictures.

.19 is returned to a suitable bias supply represented as a battery 80. The cathode 8| is grounded in this embodiment but may utilize self-bias in the well known way. The screen grid 83 is connected to a positive supply, and the plate 8B has a load resistor 86 returned to a positive supply. The plate 84 is connected directly to the plate 88 of the diode which is to clip off the unwanted components of the signal. Therathode 0| has a load-resistor.92 connected by a slider 93 to a potentiometer 94 which provides variable bias by means of one end connected to ground and the other end connected to a positive potential The output signal is connected by the condenser 95 to the television equipment which follows.

Another modification of the invention is shown in Fig. 5 wherein cascaded clipping diodes are used for eliminating amplitude excursions on both sides oi a desired wave form. -In this figure, the wave form |00 is applied to the plate |02 of the diode |03. Such a wave form as this wave form |00 has been described in the Campbell Patent 2,249,942 in connection with the generation of a composite synchronizing signal consisting of short square impulses at Ahorizontal line frequency combined with a burst of high frequency carrier at a repetition rate corresponding to vertical eld frequency. In this patent is shown means for clipping 01T to eliminate the portions of signal above and below the desired signal region, but this method used the principle of overloading ampliiier tubes, which involves the non-linearity referred to elsewhere.

This earlier procedure is simplified, as shown in Fig. 5, by using two simple diodes which in practice can even be in a single envelope. The resistor |04 is connected from the plate |02 to ground and maintains this plate at its proper average operating voltage. 'I'he cathode |05 of the diode |03 is returned through the load resistor |01 to a variable bias potentiometer |08 whose adjustment determines the extent of clipping in the negative direction. The signal on this cathode |05 is represented by the wave fo'rm |09 which shows that the negative unwanted portion of the wave form |00 has been removed. The positive portion ||2 still remains, and is clipped by diode ||5 -i'n which the cathode ||6 is connected to the output signal from the cathode |05 of the iirst diode |03. The final output signal appears on plate ||1, which is returned through resistor ||8 to another source of variable bias voltage consisting of the potentiometer I9. The output signal as shown at |2| is conducted by means of lead |22 through condenser |23 to the remaining equipment.

It has been found that the flrst blanking clipping means described abo.ve' can be pre-set to take care of all 'normal ranges of signals. A second adjustable clipping diode preferably located 1arther along in the amplified chain has proved most satisfactory for adjusting the pedestal level when changes of scene are encountered in televioutput may be minimized.

This pedestal adjustment is placed It has been found that the use of diode clippers for providing the square topped blanking signal is a considerable improvement over the triode saturation type limiters, because the diode clipper fundamentally maintains signal linearity up -to the point where clipping occurs. Also the adjustment of the clipping level is readily made simply by changing the direct current bias. It is also possible with the diode clippers to adjust easily the range of signal levels to be accepted, whereas in the old methods of saturated amplifying tubes the range of signal amplitude which can .be passed is determined rather definitely by the characteristics of the triode or pentode tube which is used. Furthermore, the diode tube is extremely simple, and a single tube can be made to do the clipping action.

It is desirable in some cases to provide an automatic method of changing the average bias supplied to theclipping diode used in the block diagram 'l of Fig. 1 for use in adjusting the pedestal level. 'I'his automatic change of bias can be incorporated in connection with a photoelectric cell which integrates the average illumination of the successive frames of motion picture illm, and

- in this way the combination of the photoelectric cell and the automatic adjustment of the bias on the pedestal controlling diode truly provides a direct current insertion at the transmitter'which will follow the average brilliance of the motion picture scenes.

What is claimed is: 1.` In a television signal control circuit, means for mixing video and blanking signals, means for establishing flat top pedestals for synchroniza- 5 tion, and means comprising a diode tube for controlling and varying the amplitude of said pedestals.

2. In a television signal control circuit, means for mixing video and blanking signals, a single 1o tube clipping means for establishing flat top pedestals for synchronization, and means comprising a diode tube for controlling and varying the amplitude of said pedestals. 3. In a television signal control circuit, means 15 for mixing video and blanking signals, means for establishing at top pedestals for synchronization, and means comprising an impedance transformer and a diode tube for controlling and varying the amplitude of said pedestals.

2o 4. In a television signal control circuit, means for mixing video and blanking signals, clipping means for establishing flat top pedestals for synchronization, means comprising a diode tube for controlling and varying the amplitude of said 25 pedestals, and means comprising cascaded clip ping diodes to control the pedestal height after clipping. l V

ROBERT E. RUTHERFORD.

30 RICHARD L. CAMPBELL. 

